1. Field of the Invention
The present invention relates to a device and method for the detection and measurement of gaseous pollutants such as nitric oxide, nitrogen dioxide, sulfur dioxide, mercaptans, hydrogen sulfide and the like in a variety of gas mixtures. More particularly, the invention is directed to the detection of these pollutants in the presence of high concentrations of carbon monoxide.
2. Discussion of the Prior Art
In recent times, a greater awareness has developed regarding the dangers of air pollution, particularly in urban or industrialized areas. Amongst the principle combustion contributions to air pollution are the products of the combustion process such as carbon monoxide and sulfur dioxide, the products of reactions between nitrogen and oxygen such as nitric oxide and nitrogen dioxide, and the by-products of industrial processes such as hydrogen sulfide and methyl mercaptan. It is not surprising, therefore, that pollutant concentrations in many areas is approaching levels known to be harmful to health.
In order to meet the needs arising in connection with pollution control of NO and NO.sub.2, and other noxious gases such as H.sub.2 S, SO.sub.2, methyl mercaptan, ethyl mercaptan and the like, extensive activity has been directed to the development and production of equipment useful in solving this problem. A problem encountered in the development of such equipment is the difficulties experienced in the detection of low concentrations of these gases in the presence of high concentrations of CO, a frequently encountered situation. Consequently, although many of these gases are known to be electrochemically active, the development of electrochemical instrumentation for these gases has been hindered by their lack of selectivity when detecting in the presence of CO.
One approach taken to improve the selectivity of the electrochemical sensors for these gases in the presence of CO has been to use a gold catalyst for the sensing electrode as described, for instance, in U.S. Pat. No. 3,776,832 to Oswin et al. This approach, however, has only been partially successful. For example, typical discrimination ratios for NO.sub.2 and H.sub.2 S in the presence of CO are -1000/1 and 2000/1, respectively. (The negative signal for the NO.sub.2 /CO ratio indicates that NO.sub.2 is electro-reduced whereas CO is electro-oxidized.) Therefore, 1000 ppm CO will give a signal equivalent to -1 ppm NO.sub.2 (negative deflection on instrument meter), and 2000 ppm CO will give a signal equivalent to 1 ppm H.sub.2 S. In order to consider the practical influence of these ratios it is necessary to consider the typical ambient concentrations of 10 ppm CO, 20 ppb NO.sub.2 and 5 ppb H.sub.2 S. Ten ppm CO will give a reading equivalent to -10 ppb NO.sub.2 (a 50% error in NO.sub.2 signal) and a reading equivalent to 5 ppb H.sub.2 S (a 100% error in H.sub.2 S signal). The magnitude of these percentage errors clearly points out the shortcomings of electrochemical instrumentation employing gold working electrodes in the detection of these pollutant gases in the presence of CO.